1. Field of the Invention
The present invention relates to nonwoven materials that have superior acoustic and thermal insulation characteristics. The invention further relates to processes for the production of such nonwoven materials. The invention further relates to airlaid structures which provide a reduced, controlled airflow therethrough useful for acoustic insulation. Still further, the invention pertains to the acoustic insulation of automobiles.
Certain aspects of the invention relate to the manufacture of acoustic panels and padding for automobiles, such as dash panel liners or mats, engine side firewall insulation, engine side hood insulation, interior wheel well insulation, trunk compartment trim insulation, flooring underlayment, package trays, and door panels. The present invention further relates to sound insulation for major appliances, such as dishwashers and washing machines, and sound and thermal insulation of exterior and interior walls, ceilings, and floors of buildings.
2. Description of the Related Art
The interior of an automobile is desirably insulated from annoying sounds, which may be transmitted through the frame and body of the automobile. These sounds typically originate from the tires as they interact with the road surface, from external wind, or from the operation of the engine and other mechanical parts of the automobile. These sounds have frequencies ranging from a few Hertz (Hz) up to several thousand Hz.
A variety of materials have been used in the fabrication of acoustic blankets. The blankets are designed and configured to be installed against surfaces of structural panels of an automotive vehicle. The insulating blankets, or pads, serve to dampen, block, absorb or attenuate unwanted road noise and external sounds. Most commonly, these blankets are comprised of recycled textile fibers formed into a material called shoddy. In some instances, foam materials may be used.
While those skilled in the art recognize that a sound absorber is most effective at a thickness corresponding to one fourth of the wavelength of the frequency of sound to be absorbed, practical considerations of space and cost may limit the actual thickness of the insulating composites which can be employed. With shoddy used in automobiles, a practical upper limit of insulating composite thickness is often thought to be approximately 25 mm (one inch) since the shoddy tends to be dense and heavy. Therefore, it is known in the art to apply an acoustical insulating barrier, sometimes called a heavy layer or viscoelastic layer, to the shoddy material for overall improved efficiency of sound reduction. Such a barrier material may double as carpeting attachment, or be included in the carpet fabrication. See, for example, U.S. Pat. Nos. 4,056,161; 4,966,799; 5,266,143; 5,068,001; and 6,109,389. Asphalt compositions, which are highly filled with dense inert powder, usually of a mineral nature, are applied in a molten state as disclosed in U.S. Pat. No. 3,429,728. Thermosetting resins like melamine, phenol-aldehydes, and urea resins are taught in U.S. Pat. No. 3,536,557, and dense filled vinyl plastisols are disclosed in U.S. Pat. No. 4,035,215. A variety of thermosetting and thermoplastic barrier materials are used in U.S. Pat. No. 4,131,664 to create the heavy or dense barrier layer. Also, a polymeric sound blocking material is disclosed in U.S. Pat. No. 3,424,270, which is hereby incorporated by reference in its entirety.
A drawback to the acoustical dampening materials disclosed in many of these patents is that they contribute significant weight to the vehicle. Those skilled in the art of acoustics understand that the best sound barrier is oftentimes a heavy, dense material like lead sheeting. However, a few pinholes or cracks can compromise even a thick or heavy sound barrier.
In lieu of sound barriers, sound absorbers, have been used. Sound absorbers typically are significantly less dense than barrier materials, and may actually be quite porous. As a result, their acoustic performance is less affected by pinholes. In addition to absorbing the sound energy, other mechanisms of reducing the perceived sound are to dampen and to block the sound waves. Although structural insulation requirements differ from automotive, the technical learnings of the one apply directly to the other.
In the final analysis, the actual physical mechanism of sound reduction (blocking or absorption) does not matter. The human ear or even a microphone cannot tell if a transmitted sound has been partially blocked or partially absorbed. In applications with numerous penetrations of the acoustic and structural panels, as in the firewall of an automobile, a sound-absorbing material may actually outperform a barrier material since the gaskets around the penetrations must be nearly perfect for the barrier material to be highly efficient in blocking the incident sound.
Routinely in the manufacture of vehicles, fibrous panels are die cut and/or molded under heat and pressure to impart a shape-sustaining contour to uniquely conform to the sheet metal of each make and model of vehicle. The molding operation can involve a heated die and cold material or the acoustic material itself is heated and then pressed in a cold die.
Criteria in the manufacture and use of sound absorbing and blocking composites, pads, baits, or blankets have been the cost of the textile raw material itself, the cost of processing into heavyweight nonwoven blankets, and the ease by which such blankets can be custom-molded to fit precisely against the structural panels of the vehicle. Other technical parameters of importance have been the acoustical properties of such fibrous nonwovens, their weight, and their durability over prolonged service during which time they may be subjected to wide variations in heat and humidity and quite possibly exposure to solvent or water-based adhesives.
Reducing the size and weight of vehicles has long been known to be effective in improving gas mileage. However, there have not heretofore been economically viable options for incorporating lighter weight acoustic insulation materials while retaining the expected level of sound-insulating performance. It is proposed here to offer nonwoven structures as a lighter alternative for acoustic insulation. In addition, it is proposed to offer nonwoven structures that provide a reduced, controlled airflow therethrough. In one aspect, the structures are airlaid structures.